Coral reefs start out small and grow about half an inch
a year, but if undisturbed by man they can grow to be quite large. The Great
Barrier Reef off the northeast coast of Australia is 150 km (93.2 mi)
wide and 2000 km (1,242 mi) long! The tan parts of the reef are the shells
of dead coral
polyps and the colorful parts are the living parts of the reef.

Reefs grow best in sunny, shallow, clear water. The water
must be clear and shallow so that the reef can get lots of sunlight. They
rarely grow deeper than 40m and they prefer salt water, doing poorly in areas
where there is a lot of river runoff due to the freshwater as well as the
silt which can cover a reef or muddy the water blocking the sunlight. The
best temperature for coral reefs is between 25 and 31û C and the best
salinity
is between 34 and 37 parts per 1000. The appropriate temperatures and salinities
are most often found in the tropics.

Reefs grow best in sunny, shallow, clear water. The water
must be clear and shallow so that the reef can get lots of sunlight. They
rarely grow deeper than 40m and they prefer salt water, doing poorly in areas
where there is a lot of river runoff due to the freshwater as well as the
silt which can cover a reef or muddy the water blocking the sunlight. The
best temperature for coral reefs is between 25 and 31û C and the best
salinity is between 34 and 37 parts per 1000. The appropriate temperatures and salinities
are most often found in the tropics.

Fringing reefs grow in shallow water along the shore and
prefer arid climates with limited river runoff. Fringing reefs are platforms
that are continuous with the shore. That means they grow right up to the edge
of the shore, like in the picture above.

Barrier reefs are separated from the shore by a wide, deep
lagoon.
They grow only when there has been a change of sea level on the adjacent coast.
This occurs when a fringing reef grows upwards so that it can stay near the
surface of the water. Coral reefs like to grow just below the waterline so
that they have the best access to the sunlight. It is best if a reef grows
at the same rate as the rise of the water. Barrier reefs also grow where the
land is sinking faster than the water. The number of barrier reefs has increased
dramatically as the
greenhouse effect has warmed up our atmosphere causing the water levels to rise.

Atolls are
reefs surrounding a lagoon. They are created when an island surrounded by barrier reefs sinks below the ocean surface,
leaving a circular reef, called an atoll. Atolls are
also created when water levels rise to cover an island and the surrounding
reef grows to keep up with
the surface of the water.